Commonly used fluorescent lights are typically long, linear, hollow glass tubes filled with reduced-pressure gases that, when excited by a suitable electrical current, cause a glow discharge (i.e., a plasma). This glow discharge produces short-wavelength light, which in turn causes fluorescence of a coating (typically lining the inside surface) within the tube. The light produced, generally white for ordinary purposes, is emitted from the coating in all directions as a diffuse glow. Because it is emitted in all directions, fluorescent light is difficult to focus to a desired spot, requires a reflector or other optical element that causes substantial losses of useful light, and requires support electronics (often referred to as a ballast) which add cost and reduce the overall efficiency. Nonetheless, fluorescent lights are ubiquitous due to their high electrical-to-optical efficiency.
Plasmas exhibit a peculiar property known as “negative resistance,” i.e., plasmas do not obey Ohm's Law—they do not exhibit a linear and positive current-voltage relationship. Instead, when an increasing voltage is applied to a fluorescent light, very little current flows until a breakdown of the gas occurs, reducing the apparent resistance, after which current and voltage increase, but usually in a non-linear fashion. The ballast, either magnetic or electronic, causes a high voltage to be initially applied to form the plasma, but thereafter limits the current to a suitable value. However, these electronics add bulk and cost, reduce efficiency, and increase the probability of failure.
Light-emitting diode (LED) technology offers a variety of advantages when compared to fluorescent and incandescent lights, including increased efficiency. When compared to fluorescent lights, LEDs differ markedly in their requirements. They require low, preferably DC voltages, typically operate at low temperature, ordinarily below about 100° C., and generally utilize a constant current for efficient operation. However, unlike fluorescent lights, LEDs are near point sources of light. There are ways to diffuse light from an array of LEDs, such as the utilization of holographic diffusers, but these may not be optimal for all applications.
Fluorescent lights typically have an energy efficiency of 50-100 lumens per watt, although the ballast and the optical efficiency of the fixture generally lower that considerably. LEDs are more efficient, e.g., in the range of 100-200 lumens per watt. And, the more directional nature of the LED light may be utilized to avoid optical inefficiencies. The use of LEDs may also obviate the need for a ballast, further improving the overall efficiency.
Linear illumination devices (i.e., those having one dimension much larger than another perpendicular dimension) incorporating LEDs typically utilize a linear arrangement (i.e., along the axial length of the device) of LEDs in individual packages including, e.g., electrical leads and focusing optics such as lenses. Each package may contain one or more individual semiconductor dies in a series, parallel, or series-parallel electrical circuit. These arrangements are capable of yielding a high lumen output but suffer from several disadvantages. First, the use of lensed LEDs causes the light source to emit a circular light beam unless the length of the LED array is considerably larger than the diameter of the beam at the desired working distance. For example, an LED with an angular distribution θ (theta) of 10°, projected from a ceiling height of 10 feet, yields a circular light with a diameter of 5.8 feet. Therefore, the length of the linear light source, and hence the length of the LED array itself, must be comparable to this diameter before the light pattern can be considered “linear.”
Furthermore, the use of large numbers of individual LEDs, while useful insofar that the heat from each LED is spatially distributed, raises the cost of the overall product because one must pay for the packaging of each LED. In addition to imposing a significant cost burden, a typical linear light source based on LEDs appears to be composed of numerous extremely bright point sources, rather than a unitary source emitting light uniformly across its length, which is distracting and results in deleterious glare. While the LEDs may be covered with a diffusing screen to “blend” the light, this diminishes optical efficiency.
Moreover, as development of LEDs matures, the light output and energy efficiency of individual LEDs increases. Thus, over time, fewer individual LEDs will be required to produce a given level of illumination. While this trend may reduce overall cost, it also implies that a desired light output necessitates a linear array of fewer LEDs. For a given linear light source dimension, the LEDs will be spaced further apart, further confounding the shape of the resulting light pattern and exacerbating the above-described issues.
In order to address these problems, light guides have been developed to transform the light in a beneficial manner. In general, such light guides provide illuminating light from the long dimension of a clear solid or hollow rod while light enters the rod from the small dimension (i.e., the “end”). Unfortunately, the optical efficiency of such devices is very low −50% or more of the light generated by the external light source is lost (i.e., not emitted from the light guide), making such light guides undesirable as means of providing high efficiency, compact, linear lighting.
FIG. 1 depicts one origin of inefficiencies in such an optical assembly 100. Generally, an LED 110 faces the flat face 120 of the light guide 130. Light emerging from the LED 110 generally has a Lambertian (i.e., omnidirectional) distribution. An exemplary light ray 140, representing a portion of the light emitted near the plane of the LED (i.e., nearly parallel to face 120), does not impinge upon the flat face 120 and is lost. Light emitted toward the light guide 130 at a slightly greater angle, e.g., light ray 150, will be at least partially reflected at the flat face 120, although a portion of the ray 150 will typically enter the light guide 130. And, since such light enters the light guide 130 at an angle greater than that required for total internal reflection (TIR), this light will still emerge from the light guide 130. However, because this light will be cut off abruptly when the condition for TIR occurs (whether or not it is reflected from the back surface of the rod (i.e., a reflector 160), as shown in FIG. 1), the exiting light ray 170 will form an undesirable localized spot in the light emitted from light guide 130. (By way of example, the sharp cutoff for a plastic rod of refractive index of 1.49 in air occurs at 42°.) In contrast, the light ray 180 is internally reflected toward reflector 160. Additionally, light that is not reflected from the reflector 160, and which arrives at the far surface 190 of the light guide 130 at an angle greater than that required for total internal reflection will exit the light guide 130 (except for a small amount reflected back at the surface 190 (not shown)), further reducing the efficiency of the optical assembly 100.
Thus, there is a need for linear illumination devices based on LEDs that simulate the light pattern emitted by fluorescent lights while limiting light losses and localized spots in the emission pattern, and that satisfy the different operating requirements for LEDs while enabling them to function with high efficiency.